1. Field of the Invention
This invention relates to the structure of an optical head, and more particularly to the structure of an optical head for an opto-magnetic recording medium in which the reflected light from the recording medium is received by a photodetector such as a photoelectric element and the positional deviation relative to the optical head is detected by a variation in the quantity of light or the shape of the light beam and focusing control (automatic focus control) or tracking control (automatic pursuit control) is effected on the basis of the electrical signal resulting from the detection.
2. Description of the Prior Art
As examples of the recording medium in which a minute light spot is condensed to thereby accomplish recording or reproduction, there are video discs and digital audio discs on which signals such as images or sounds are pre-recorded and magneto-optical recording mediums having a layer of recording material varied by light or the heat energy by light.
The former recording mediums are used only for the reproduction of signals, while the latter recording mediums permits also recording of signals on the part of the user. Also, magneto-optical recording mediums are convenient in that they permit signals to be erased to repetitively record and reproduce signals.
Where signals are recorded or reproduced by causing a minute light spot to be imaged on such a recording medium while moving the recording medium as by rotation or vibration, vertical vibration and lateral displacement due to surface vibration occur to the recording medium and there is caused a relative positional fluctuation between the optical head on which the light spot is imaged and the recording medium. As a result, the size of the spot on the recording medium fluctuates or the applied position of the spot deviates from the signal track and the signals picked up and the pattern recorded become unclear.
To solve this problem, it has been practiced to receive the reflected light from the recording medium by a photoelectric detector, detect the relative positional deviation of the optical head and the recording medium by a variation in the quantity of light or the shape of the light beam and feed back a focus error signal or a tracking error signal to a part of the optical head or of the component of the optical head, and correct the spacing therebetween and the positional deviation such as lateral displacement on the basis of said signal.
An example of the structure of such a conventional optical head will hereinafter be described by reference to FIGS. 1 to 3 of the accompanying drawings.
In FIG. 1, a condensing lens 2 for condensing a light spot on a recording medium 1 is mounted for movement in the direction of the optical axis by drive means 3. Reference numeral 4 designates a pivoting mirror for deflecting the light beam, reference numeral 5 denotes a quarter wavelength plate, reference numeral 6 designates a collimater lens, reference numeral 7 denotes a polarizing beam splitter, reference numeral 8 designates a light source such as a semiconductor laser, reference numeral 9 denotes a cylindrical lens, and reference numeral 10 designates a four-division photodetector.
The light beam emitted from the light source 8 passes through the polarizing beam splitter 7 and is made into a parallel light beam by the collimater lens 6. In this case, the plane of polarization of the light beam emitted from the light source 8 is adjusted so as to be parallel to the plane of the drawing sheet, and the polarizing beam splitter 7 passes therethrough the light beam of such a plane of polarization substantially without any loss. The parallel light beam becomes a circularly or elliptically polarized light beam by passing through the quarter wavelength plate 5 and subsequently is reflected by the pivoting mirror 4 rotatable about its pivot, whereafter it is condensed as a minute spot on the recording medium 1 by the condensing lens 2.
On the other hand, the reflected light beam from the recording medium 1 arrives at the quarter wavelength plate 5 via the condensing lens 2 and the pivoting mirror 4. The light beam passed through this quarter wavelength plate 5 has its plane of polarization orthogonal to that during its incidence and almost all of this light beam is reflected by the polarizing beam splitter 7 and enters the four-division photodetector 10 via the cylindrical lens 9. In this case, the reflected light forms a light beam in the form of astigmatism distribution on the photodetector by an astigmatism producing optical system comprised by the collimater lens 6 and the cylindrical lens 9. Accordingly, the focus state of the spot on the recording medium 1 can be detected from the state of its distribution.
FIGS. 2A, 2B and 2C show the states of distribution of light on the photodetector 10 in various focus states of the spot, FIG. 2A showing the distribution of light in the front focus state, FIG. 2B showing the distribution of light in the in-focus state, and FIG. 2C showing the distribution of light in the rear focus state.
If the respective photoelectric elements of the four-division photodetector 10 are A, B, C and D as shown in FIGS. 2A-2C, the output value of (A+C)-(B+D) can be detected on the basis of the quantity of light received by each element and the front focus state, the in-focus state or the rear focus state can be discriminated by whether the output value is negative, zero or positive. Also, by feeding back this output value to the drive device 3 through an electrical processing system having a suitable gain, correction of the fluctuation of the distance between the condensing lens and the recording medium 1, i.e., automatic focus control (focusing control) can be effected.
The automatic pursuit control (auto tracking) for properly pursuring the signal track of the recording medium is carried out in the following manner.
That is, as shown in FIG. 2D, the four-division photodetector 10 is disposed so that the division line thereof is along the direction T-T' of the track indicated by broken line.
If the minute spot deviates from the signal track under the influence of the eccentricity or the like of the recording medium 1 which is a rotating member, inclination occurs to the distribution of intensity of the light beam as shown in FIG. 2D and accordingly, the track deviation can be detected as a variation in the output value of (A+D)-(B+C) from the quantity of light received by each light-receiving element. The signal indicative of this track deviation, as in the case of the aforementioned automatic focus control, is electrically processed and then fed back to the driving system 3 of the pivoting mirror 4, whereby automatic pursuit control (auto tracking) becomes possible.
In the conventional auto focusing and auto tracking described above, if the focus length of the condensing lens 2 is f, the spot on the recording medium 1 can be laterally moved by 2f.theta. by rotating the pivoting mirror 4 by .theta. as shown in FIG. 3A. However, when the reflected light beam arrives at the four-division photodetector 10 through the condensing lens 2, the pivoting mirror 4 and the cylindrical lens 9, the optic axis is deviated by the aforementioned lateral movement of the light spot, and the distribution of light on the photodetector 10 moves from the circle indicated by dot-and-dash line to the circle indicated by broken line, as shown in FIG. 3B.
Then, the variation in the output value of (A+D)-(B+C) is very slight if the distribution of light is in the form of origin-symmetrical distribution and moreover is the movement along the division line, but actually the optical system is one using the cylindrical lens 9 and therefore, neither of the movement and shape of the distribution of light becomes ideal. Accordingly, the movement of the light beam caused by the afore-mentioned tracking operation must be kept to a degree which does not impart influence to the focus signal, and this leads to a problem that the range of tracking control is limited.
That is, in the above-described conventional optical head, when the pivoting mirror 4 is vibrated to effect tracking, there is a disadvantage that the focus signal is also affected and accurate automatic focus control becomes difficult and a disadvantage that alignment of the four-division photodetector 10 and the cylindrical lens 9 must be strictly effected and therefore much time is required of the assembly of the optical head and the cost of manufacture increases.
As a conventional method for eliminating such disadvantages, there has been proposed, for example, the Fouco method (knife edge method) whereby a focus signal is obtained and the photodetector is disposed at a position conjugate with the light source (the position indicated at 11 in FIG. 3A) to solve the problem that the light beam is deviated during the tracking, but in the focus detection by this Fouco method, the light beam is intercepted by the knife edge in the optical path, and this leads to a disadvantage that loss of the quantity of light is involved.